docs: Created migration guide from SSD -> Distributed

[Jayclifford345]

What this PR does / why we need it:
This PR includes a migration guide from Loki SSD to Loki distributed for users using the Loki helm.

I would also like to give credit to @lindeskar for initially running through this process in the dark:
#13726

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[github-actions]

💻 Deploy preview available: https://deploy-preview-loki-16389-zb444pucvq-vp.a.run.app/docs/loki/latest/

[lindeskar]

Really nice guide, thanks for putting it together!

[lindeskar]

Should this be removed after a finished migration?

[Jayclifford345]

Good point, I can leave it as optional. It depends if the operator wants to force ingesters to flush on a shutdown. I will ask Ed for his opinion aswell

[mwtzzz]

Hi, going through your doc today, the only problem I have noticed so far, is that the ruler sidecar is not migrated properly.

If using a SimpleScaleable mode with a config like the following:

loki:
  rulerConfig:
    enable_alertmanager_v2: true
    alertmanager_url: http://xxx
    storage:
      type: local
      local:
        directory: /rules
sidecar:
  rules:
    enabled: true
    folder: /rules/1
    logLevel: debug

Then when applying the SSDM->Distributed migration, there are two problems with the new ruler pod:

1. the new ruler pod does not contain any sidecar
2. the new ruler pod throws an error:

level=error ts=2025-02-21T21:11:16.53272825Z caller=log.go:216 msg="error running loki" err="mkdir /rules: read-only file system